1. Field of Invention
This invention relates to a storage medium which displays images (including information such as characters, graphics and data) and stores a display status thereof. This invention also relates to a method and apparatus for writing images to the storage medium, a method of forming the storage medium and a storage medium formed using the method.
2. Description of Related Art
Bulk consumption of paper around offices results in destruction of forest resources and environmental pollution resulting from refuse disposal and incineration. However, with widespread use of personal computers and the advent of our advanced information-intensive society brought about by the Internet and the like, the trend is moving toward more and more consumption of paper as so-called short-life documents intended for temporary browsing of electronic information, so that the advent of display storage media to replace paper is desired.
Paper has the following advantageous information display characteristics not found in conventional displays: 1) paper is capable of a bright and high-contrast reflective full-color display, is easy to read, and displays large quantities of information; 2) paper is structurally lightweight, thin, and flexible and therefore, is viewable in a comfortable position and under desirable brightness; 3) paper has memory capability, can display and store information without power, and provides a flicker-free and easy-on-the-eyes display; and 4) paper is inexpensive and facilitates an easy understanding of simultaneous displays of a plurality of sheets of paper, thereby allowing information comparison and browsing. These characteristics of paper, in turn, drive users to print information displayed on a display unit on paper before reading it.
Therefore, there is a need for display storage media to replace paper that have the above-mentioned paper-specific characteristics with the addition of a rewritable capability which would contribute to resource saving and reduction in refuse.
A display method, generally called NCAP, disclosed in Japanese Patent Publication No. Hei 3-52843, which uses droplets of nematic liquid crystal having positive dielectric anisotropy dispersed in a polymer matrix as shown in FIG. 23, has been proposed. With this method, the following two states are produced and a black and white monochrome display is obtained by switching between the states and providing a light absorption layer on a non-display surface: (1) an initial state as shown in FIG. 23(A) in which liquid crystal directors orient in random directions under the influence of the surface of the polymer matrix and incident light is scattered due to mismatch of refractive indexes between the polymer matrix and the liquid crystals; and (2) a state as shown in FIG. 23(B) in which the liquid crystal directors orient in a field direction by applying a voltage and incident light transmits due to match of refractive indexes between the polymer matrix and the liquid crystals.
However, this method has a drawback in that, when a white display is performed by back scattering resulting from mismatch of refractive indexes between the polymer matrix and the liquid crystals, a reflectance of only 10 to 15% is obtained because the difference of refractive indexes between the polymer and the liquid crystal is not so large and a sufficient scattering is not obtained. Thus, a bright and high-contrast display cannot be obtained.
To overcome this drawback, dyes have been added to the liquid crystal to improve brightness and contrast. However, this method has a serious drawback in that there is no memory capability in transparent state of a black display and no information can be displayed and saved without power.
On the other hand, as a method of performing a display with memory capability, for example, there is known a method by which texture change is caused by heat and a field using a smectic liquid crystal as shown on Page 456 of the Liquid Crystal Device Handbook (published by Nikkan Kogyo Shimbun).
According to this method, the following two states are produced and a black and white monochrome display is obtained by switching between the states and providing a light absorption layer on a non-display surface: (1) a state in which incident light is scattered by a focal conic texture obtained by cooling electric-field-free a smectic liquid crystal heated to an isotropic state as shown in the upper portion of FIG. 24 as shown in FIG. 24(A); and (2) a state in which incident light is transmitted by a homeotropic texture obtained by cooling a smectic liquid crystal heated to an isotropic state as shown in the upper portion of FIG. 24 while applying a field as shown in FIG. 24(B).
However, this method has a drawback in that, although information can be displayed and saved without power, a white display is performed by the disorder of the layer structure of a smectic liquid crystal from which sufficient back scattering is not obtained, and the reflectance of the white display is low like the above-mentioned NCAP. Thus, a bright and high-contrast display cannot be obtained.
According to a method by which a display is performed while switching between transparent state and scattering state, such as conventional examples shown in FIG. 23 or 24, a color display can be performed using an xe2x80x9carray systemxe2x80x9d which arranges pixels producing different colors on a display surface, such as a method of combination with color filters, for example.
However, the array system has a drawback in that, since it requires exact alignment of pixel portions each producing a different color, and addressing portions writing data corresponding to each color, it is difficult to write an image to a display element from the outside. It is necessary to provide electrodes subjected to patterning and the like within the display element. Thus, the display element becomes costly.
On the other hand, there are proposed several color display methods using a xe2x80x9cstack systemxe2x80x9d which arranges elements producing different colors in an observation direction. For example, according to a method disclosed in Japanese Published Unexamined Patent Application No. Hei 3-209425, as shown in FIG. 25, three display layers 38A, 38B, and 38C selectively reflecting blue, green, and red, respectively, are stacked between a pair of substrates 32 and 33 such that droplets of cholesteric liquid crystal 42 having negative dielectric anisotropy are dispersed in a polymer matrix 41 within each display layer. A separation substrate 34 intervenes between the display layers 38A and 38B and a separation substrate 35 intervenes between the display layers 38B and 38C. A light absorption layer 36 is provided on a non-display surface and drive electrodes 37 subjected to patterning of each of the display layers 38A, 38B, and 38C are connected to a drive circuit 50 comprising a drive power supply and a switch 52.
According to this method, the following two states are produced and a color display is obtained by switching between the states: an initial state in which liquid crystal directors orient in random directions under the influence of the surface of the polymer matrix 41 and incident light almost transmits; and a state in which the helical axes of cholesteric liquid crystals 42 are oriented in a field direction by applying a voltage and a specified color in incident light is selectively reflected.
Further, xe2x80x9cReflective Cholesteric Liquid-Crystal Displays,xe2x80x9d Information Display, pages 18-21, December 1996, describes the use of cholesteric liquid crystals having positive dielectric anisotropy in a color display element of the stack system, as shown in FIG. 25, is described.
However, since these methods require that the stacked display layers 38A, 38B, and 38C be switched separately, it is impossible to write an image to the display element from the outside. Accordingly, it becomes necessary to provide drive electrodes 37 subjected to patterning for each of the display layers 38A, 38B, and 38C within a display element, so that disadvantageously the display element becomes costly.
A display method by which droplets of cholesteric liquid crystal having negative dielectric anisotropy are dispersed in a polymer matrix and texture change is caused by heat and a field is shown in Japanese Published Unexamined Patent Application No. Hei 6-258622.
According to this method, as shown in FIG. 26, in a color display element of the stack system as shown in FIG. 25, plane drive electrodes 39 common to pixels, are provided in each of the display layers 38A, 38B, and 38C in place of drive electrodes subjected to patterning. The following two states are produced and a color display is obtained by switching between the states: (1) a state in which incident light is almost transmitted by a random state obtained by cooling electric-field-free cholesteric liquid crystals 42 in each of the display layers 38A, 38B, and 38C, the cholesteric liquid crystals 42 having been heated to an isotropic state by a combination of heat from a heat source, such as laser shown in FIG. 60, and application of a field by a drive circuit 50; and (2) a state in which a specified color in incident light is selectively reflected by a planar texture obtained by cooling the cholesteric liquid crystals 42 heated to an isotropic state while applying a field.
However, this method also has a drawback in that the drive electrodes 39, which are not ones subjected to patterning but plane ones common to pixels, must be provided for each of the display layers 38A, 38B, and 38C within a display element. Thus, the display element becomes costly like the method shown in FIG. 25. Further, it takes a long time to write an image because it is written while performing heating and cooling for each pixel by a heat source, such as laser.
In contrast to these methods, there are also proposed several methods which enable an image to be written to a display element from the outside by switching among a plurality of display layers using only one drive signal without providing drive electrodes for each of the display layers within a display element in a color display element of the stack system.
For example, in Japanese Published Unexamined Patent Application Nos. Hei 3-198028 and Hei 6-265854 is shown a method by which a plurality of display layers, each with dichroic dyes mutually different in absorption wavelength range being contained in a liquid crystal, are stacked, and a plurality of colors are displayed by only one drive signal using a difference of threshold voltage among liquid crystals in each display layer.
Further, in Japanese Published Unexamined Patent Application No. Hei 5-31315 is shown a method by which a plurality of liquid crystal microcapsules containing dichroic dyes different in absorption wavelength range are mixed and a plurality of colors are displayed by only one drive signal, using a difference of threshold voltage among liquid crystals of each capsule.
However, according to these methods, for example, a plurality of display layers or capsules, shown as layers A, B, and C in FIG. 27(A), whose display state changes unidirectionally, such as from dark off-state to bright on-state in response to applied voltage, are used. Since the threshold voltage of switching among the display layers or capsules is changed, even when the voltage Va, Vb, Vc, and Vd between the threshold values of the display layers or capsules are applied, control can be performed only so that the display layers or capsules change to the bright on-state in ascending order of threshold voltage. Thus, it is impossible to perform control so that only a particular display layer or capsule is driven into the bright on-state.
Accordingly, all three primary colors, blue, green, and red, or cyan, magenta, and yellow, cannot be displayed even if the principle of additive color mixture or subtractive color mixture is used. Thus, these methods have the drawback of being incapable of full color display.
As described above, in an attempt to produce paper-like display storage media by traditional technologies, a drawback with display storage media of monochrome display is that a bright, high-contrast display cannot be obtained because of the low reflectance in a white display. A drawback with display storage media of a color display is that it is impossible to write full-color images by external equipment because a plurality of display layers for displaying the three primary colors cannot be controlled simultaneously and independently by only one drive signal.
In view of the above, the present invention provides a storage medium which has memory capability without power. The storage medium is capable of a bright and high-contrast black-and-white reflective monochrome display or reflective full-color display, allows images to be written and updated quickly by external equipment, is structurally lightweight, thin, and flexible, and is inexpensive to manufacture. The present invention also provides a method and an apparatus which enables images, i.e. characters, graphics, data, and the like, to be written to the storage medium from outside the storage medium.
According to a first aspect of the storage medium, a plurality of display layers, which selectively reflect mutually different wavelengths of the electromagnetic spectrum, are stacked between a pair of substrates at least one of which is transparent. An image is written in such a manner that a voltage is applied to the plurality of stacked display layers from an external image writing apparatus.
The present invention, as a second aspect, provides a storage medium wherein the plurality of display layers have different texture change threshold voltages for voltage applied from an external image writing apparatus.
As a third aspect of the present invention the plurality of display layers comprise a display layer selectively reflecting visible light having a peak in a wavelength range of 400 to 500 nm, a display layer selectively reflecting visible light having a peak in a wavelength range of 500 to 600 nm and a display layer selectively reflecting visible light having a peak in a wavelength range of 600 to 700 nm.
As a fourth aspect of the invention, the plurality of display layers each have a PNLC structure in which a polymer network is formed in a continuous phase of cholesteric liquid crystals.
As a fifth aspect of the invention, the plurality of display layers each have a PDLC structure in which cholesteric liquid crystals are dispersed in a polymer matrix.
As a sixth aspect of the invention, the plurality of display layers each comprise two display layers which selectively reflect a mutually identical color of visible light and are opposite in helical twist direction to each other.
As a seventh aspect of the invention, the pair of substrates have flexibility.
As an eighth aspect of the invention, a common electrode is provided in one substrate.
The present invention also provides a method of writing images to a storage medium in which a write signal is applied to a plurality of stacked display layers from an external image writing apparatus using a voltage Vs, wherein Vs is an applied voltage in a select stage of the write signal and causes all the cholesteric liquid crystals of the plurality of display layers to change to the same alignment state. The write signal consists of at least the select stage and a subsequent voltage-free display stage.
The present invention also provides a method of writing images to a storage medium in which a write signal is applied to the plurality of stacked display layers from an external image writing apparatus with a voltage having a relation of Vr greater than Vs, wherein the voltage is selected from a plurality of levels of voltage which are produced based on the respective threshold voltages of cholesteric liquid crystals of the plurality of display layers. Vr and Vs are the applied voltages in a refresh stage and a select stage of the write signal, respectively, which includes the refresh stage, the select stage, and a subsequent voltage-free display stage.
The present invention also provides an apparatus for writing images to a storage medium through which a write signal is applied to a plurality of stacked display layers from the outside of the storage medium using a voltage Vs, wherein Vs is an applied voltage in the select stage of a write signal and causes all the cholesteric liquid crystals of the plurality of display layers to change to the same alignment state. The write signal consists of at least the select stage and a subsequent voltage-free display stage.
The present invention also provides an apparatus for writing images to a storage medium through which a write signal is applied to the plurality of stacked display layers from the outside of the storage medium with a voltage having a relation of Vr greater than Vs, wherein the voltage is selected from a plurality of levels of voltage which are produced based on the respective threshold voltages of cholesteric liquid crystals of the plurality of display layers. Vr and Vs are respectively the applied voltages in a refresh stage and a select stage of a write signal, which includes the refresh stage, the select stage, and a subsequent voltage-free display stage.
In the description of the present invention, the term xe2x80x9ccholesteric liquid crystalxe2x80x9d includes chiral nematic liquid crystal, or chiral smectic liquid crystal and the like.
Cholesteric liquid crystals having liquid crystal molecules of helical structure cause a selective reflection phenomenon. That is, they divide incident light into right circularly polarized light and left circularly polarized light, and Bragg-reflect circularly polarized light components matching a helical twisting direction and transmit residual light components. Letting a helical pitch length, an average refraction index within a plane orthogonal to a helical axis, and birefringence be p, n, and xcex94n, respectively, the central wavelength xcex of reflection light and reflection wavelength width xcex94xcex are represented by nxc2x7p and xcex94nxc2x7p, respectively, so that reflection light caused by cholesteric liquid crystal layers develop vivid colors dependent on the helical pitch length.
Cholesteric liquid crystals having positive dielectric anisotropy exhibit three states: (1) a planar texture as shown in FIG. 21(A), in which helical axes are perpendicular to the cell surface and the above-mentioned selective reflection phenomenon occurs for incident light; (2) a focal conic texture as shown in FIG. 21(B), in which helical axes are almost parallel with the cell surface and incident light is transmitted while being front scattered; and a (3) homeotropic texture as shown in FIG. 21(C), in which a helical structure collapses, liquid crystal directors orient in a field direction, and incident light is almost wholly transmitted.
Of the three states described above, the planar texture and the focal conic texture can exist bi-stably at zero field. Therefore, the state of cholesteric liquid crystal is not uniquely determined for a volatage applied to a liquid crystal layer. When the planar texture exists initially, texture transition occurs in the order of planar texture, focal conic texture, and homeotropic texture as voltage increases, and when the focal conic texture exists initially, texture transition occurs in the order of focal conic texture and homeotropic texture as applied voltage increases. On the other hand, when a voltage applied to a liquid crystal layer is abruptly set to zero, the planar texture and the focal conic texture stay unchanged and the homeotropic texture changes to the planar texture.
Accordingly, after a pulse signal is applied, a cholesteric liquid crystal layer exhibits a xe2x80x9cwell-typexe2x80x9d electro-optical response as shown, for example, in FIG. 22. The term xe2x80x9cwell-typexe2x80x9d electro-optical response refers to the general shape of the curve shown in FIG. 22 and is not meant to refer to the specific data points shown in FIG. 22.
As shown in FIG. 22, when the voltage of an applied pulse signal is Vfh 90 or more, selective reflection occurs as a result of texture transition from the homeotropic texture to the planar texture. When the voltage of an applied pulse signal is between Vpf 10 and Vfh 10, transparent by the focal conic texture occurs. When the voltage of an applied pulse signal is Vpf 90 or less, the state before the pulse signal was applied occurs. That is, selective reflection by the planar texture or transparent by the focal conic texture occurs.
In FIG. 22, the vertical axis, which represents normalized reflectance, normalizes reflectance with the maximum reflectance as 100 and the minimum reflectance as 0. Since transition regions exist among the states of the planar texture, focal conic texture, and homeotropic texture, the portion of normalized reflectance of 90 or more is defined as selective reflection state and the portion of normalized reflectance of 10 or less is defined as transparent state. The threshold voltages of texture change from the planar texture to the focal conic texture are defined as Vpf 90 and Vpf 10 before and after the transition region, respectively, and the threshold voltage of texture change from the focal conic texture to the homeotropic texture are defined as Vfh 10 and Vfh 90 before and after the transition region, respectively.
Particularly, in display layers of PNLC or PDLC structure to which a polymer is added to cholesteric liquid crystals, because of interference (anchoring effect) in the interface between a cholesteric liquid crystal and a polymer, bi-stability of the planar texture and the focal conic texture at zero field is improved and a state after a pulse signal is applied can be kept for a long time.
In a storage medium according to this invention, using the bi-stable phenomenon of the cholesteric liquid crystal, a black-white monochrome display having electric-field-free memory capability or a color display having electric-field-free memory capability is performed by switching between selective reflection by the planar texture and transparent by the focal conic texture.
A storage medium according to this invention does not require internal electrodes and wiring for applying a voltage or one that has only electrodes common to pixels in one substrate. In the case of the former, an image is written to the storage medium by applying a voltage between a pair of substrates of the storage medium from an external image writing apparatus by write electrodes which are positioned so as to sandwich the pair of substrates of the storage medium. In the case of the latter, an image is written to the storage medium by applying a voltage between the common electrode of the storage medium and another substrate from an external image writing apparatus by the common electrode on one substrate of the storage medium and an electrode included in the external image writing apparatus, the electrode being positioned outside another substrate of the storage medium.
In either case, to each display layer is applied a voltage determined by a relationship of resistance values and capacitance among components of a pair of substrates or another substrate, each display layer, and a separation layer provided as required. Since any components normally exhibit sufficiently large resistance values, the ratio of voltages to the components almost depends on the ratio of capacitance. Also, since cholesteric liquid crystals have dielectric anisotropy and electrostatic capacity changes depending on the respective states of planar texture, focal conical phase, and homeotropic texture, a voltage applied to each display layer also changes depending on the switching of any display layer.
Therefore, in a storage medium according to this invention, as shown in FIG. 22, by combining the electro-optical response for a voltage actually applied to each display layer with a voltage ratio depending on the ratio of capacitance of components of the storage medium the alignment state of each display layer for a voltage applied by an external image writing apparatus is obtained and is controlled to change to a desired state whereby each display layer can be switched simultaneously and independently.
When an image writing method is used for a storage medium, a write signal is applied to the plurality of stacked cholesteric liquid crystal display layers from an external image writing apparatus with a voltage Vs, wherein Vs is an applied voltage in a select stage Ts of a write signal and causes all the plurality of cholesteric liquid crystal display layers to change to the same alignment state. The write signal consists of at least the select stage Ts and a subsequent electric-field-free display stage Td, whereby (1) all the plurality of cholesteric liquid crystal display layers are changed to the planar texture state, or (2) all the plurality of cholesteric liquid crystal display layers are changed to the focal conic texture state.
Accordingly, the following three cholesteric liquid crystal display layers, for example, are stacked: a layer selectively reflecting blue light having a peak in a wavelength range of approximately 400 to 500 nm; a layer selectively reflecting green light having a peak in a wavelength range of approximately 500 to 600 nm; and a layer selectively reflecting red light having a peak in a wavelength range of approximately 600 to 700 nm. A light absorption layer is provided opposite to an external light incident side, whereby (1) three cholesteric liquid crystal display layers all become the selective reflection state and white is displayed by additive color mixture, or (2) the three cholesteric liquid crystal display layers all become the transparent state and light transmitting through the three cholesteric liquid crystal display layers is all absorbed in the light absorption layer and black is displayed, so that two colors, black and white, can be displayed within one pixel.
Since a maximum reflectance of 50% is obtained in a peak wavelength range in the selective reflection phenomenon by cholesteric liquid crystals, a high integral reflectance can also be obtained in a white display represented by additive color mixture, so that a bright and high-contrast display can be performed.
Furthermore, a brighter display can be obtained by forming a plurality of display layers such that each of them is constituted of two display layers which selectively reflect a mutually identical color and are opposite to each other in the helical twisting direction of cholesteric liquid crystals.
A storage medium according to this invention has no internal electrodes and wires for applying a field or has a common electrode only in one substrate and requires no internal drive circuit. Thus, the storage medium it can be manufactured inexpensively and can be structurally lightweight, thin, and flexible.
In the case of using an image writing method for a storage medium when three cholesteric liquid crystal layers, are stacked, a write signal is applied to the three stacked cholesteric liquid crystal display layers from an external image writing apparatus with a voltage having a relation of Vr greater than Vs. Vr and Vs are respectively the voltages in the refresh stage Tr and the select stage Ts of a write signal, which includes a refresh stage Tr, a select stage Ts, and a subsequent electric-field-free display stage Td. The voltage is selected from seven levels of voltage which are produced based on the respective texture change threshold voltages of the three cholesteric liquid crystal display layers. One of the following four texture change states is obtained: (1) all the three cholesteric liquid crystal display layers are in the state of planar texture; (2) all the three cholesteric liquid crystal display layers are in the state of focal conic; (3) one of the three cholesteric liquid crystal display layers is in the state of planar texture and the two remaining layers are in the state of focal conic texture; and (4) two of the three cholesteric liquid crystal display layers are in the state of planar texture wherein the two layers include a layer whose threshold voltage is in the middle of them of other two layers, and the remaining layer is in the state of focal conic texture.
Accordingly, the following three cholesteric liquid crystal display layers, for example, are stacked: a layer selectively reflecting blue light having a peak in a wavelength range of approximately 400 to 500 nm; a layer selectively reflecting green light having a peak in a wavelength range of approximately 500 to 600 nm; and a layer selectively reflecting red light having a peak in a wavelength range of approximately 600 to 700 nm. A light absorption layer is provided opposite to an external light incident side, whereby (1) three cholesteric liquid crystal display layers all become the selective reflection state and white is displayed by additive color mixture, (2) the three cholesteric liquid crystal display layers all become the transparent state and light transmitting through the three cholesteric liquid crystal display layers is all absorbed in the light absorption layer and black is displayed, (3) only one of the three cholesteric liquid crystal display layers becomes the selective reflection state and red, green, or blue is displayed, or (4) only two of the three cholesteric liquid crystal display layers become the selective reflection state and two of yellow, magenta, and cyan are displayed wherein the two layers include a layer whose threshold voltage is in the middle of those of other two layers, so that a total of seven colorsxe2x80x94five colors, black, white, red, green, and blue, and two of yellow, magenta, and cyanxe2x80x94can be displayed within one pixel.
Since a maximum reflectance of 50% is obtained in a peak wavelength range in the selective reflection phenomenon by cholesteric liquid crystals, a high integral reflectance can be obtained in a display of white, yellow, magenta, and cyan represented by additive color mixture, as well as in a display of the primary colors such as blue, green, and red, so that a bright and high-contrast display can be obtained.
Furthermore, a brighter display can be obtained by forming the three display layers such that each of them is constituted of two display layers which selectively reflect a mutually identical color and are opposite to each other in the helical twisting direction of the cholesteric liquid crystals.
Furthermore, a full color display can be obtained by applying area modulation method such as dithering and the error diffusion method, using at least five colors, for example, white, black, blue, green, and red.
A storage medium according to this invention has no internal electrodes and wires for applying a voltage or has a common electrode only in one substrate and requires no internal drive circuit. Thus, the storage medium can be manufactured inexpensively and can be structurally lightweight, thin, and flexible.
As described above, according to this invention, there is provided a paper-like storage medium which has a high-speed rewrite capability by electro-optical response of cholesteric liquid crystals and memory capability without power. The storage medium is capable of a bright and high-contrast black-and-white monochrome display or full color display, is structurally lightweight, thin, and flexible, and is inexpensive to manufacture.